Back mutation
A point mutation in a mutant allele that changes it back to a wild-type allele. Also called reversion.
Bacteria Prokaryotes
Micro-organisms that constitute one of the three main evolutionary domains of organisms. Members of this domain are called bacteria.
Bacterial artificial chromosome (BAC)
A vector for cloning DNA fragments up to about 200 kb long in E. coli. A BAC contains the origin of replication of the F factor, a multiple cloning site, and a selectable marker.
Bacteriophages
Viruses that attack bacteria. Also called phages.
Barr body
A highly condensed and transcriptionally inactive X chromosome found in the nuclei of somatic cells of female mammals.
Base
Also called nitrogenous base. Purine or pyrimidine component of a nucleotide.
Base analog
A chemical whose molecular structure is very similar to that of one of the bases normally found in DNA. Some chemical mutagens, such as 5-bromouracil (5BU), are base analogs.
Base excision repair
An enzyme-catalysed process for repairing damaged DNA by removal of the altered base, followed by excision of the baseless nucleotide. The correct nucleotide then is inserted in the gap.
Base-modifying agent
A chemical mutagen that modifies the chemical structure of one or more bases normally found in DNA. Nitrous oxide, hydroxylamine, and methylmethane sulfonate are common base-modifying agents.
Base-pair substitution mutation
A change in the genetic material such that one base pair is replaced by another base pair; for instance, an A-T is replaced by a G-C pair.
Basic research
Research done to further knowledge for knowledge’s sake.
Bidirectional replication
Synthesis of DNA in both directions away from an origin of replication.
Bioinformatics
Application of mathematics and computer science to store, retrieve, and analyse biological data, particularly nucleic acid and protein sequence data.
Bivalent
A pair of homologous, synapsed chromosomes, consisting of four chromatids, during the first meiotic division.
Bootstrap procedure
A method for determining confidence levels attached to the branching patterns of a phylogenetic tree chosen by the parsimony approach.
Bottleneck effect
A form of genetic drift that occurs when a population is drastically reduced in size and some genes are lost from the gene pool as a result of chance.
Branch-point sequence
Specific sequence within introns of precursor mRNAs (pre-mRNAs) of eukaryotes containing an adenylate (A) nucleotide to which the free 5¿ end of an intron binds during mRNA splicing.
Broad-sense heritability
The proportion of the phenotypic variance within a population that results from genetic differences among individuals.
